Abstract: An apparatus for heating articles includes a main heating vessel having an inlet opening, an outlet opening and an exhaust opening therein, a quantity of a vaporizable heat transfer liquid in the vessel, a heating unit for heating the heat transfer liquid to produce a vapor blanket in the vessel, a transport assembly for transporting an article to be heated so that it passes through the vapor blanket and a control for controlling the heating unit to maintain the height of the vapor blanket at a predetermined level. The inlet opening, the outlet opening and the exhaust opening are all at least partially above the upper level of the vapor blanket, and an exhaust fan is provided for withdrawing air from the main heating vessel so that air is continuously drawn inwardly through the inlet opening and the outlet opening to prevent loss of vaporized heat transfer medium from the vessel.